This disclosure relates to an ionizing radiation stable polyarylestercarbonate composition, and especially relates to an ionizing radiation stable polyarylestercarbonate composition suitable for use in medical devices which are sterilized using ionizing radiation sterilization. It also relates to methods for making the ionizing radiation stable composition and the articles suitable for use in medical applications produced from these ionizing radiation stable compositions.
A common procedure for plastics used in medical devices is to subject them to sterilization in order to reduce the possibility of infection to patients undergoing medical procedures. Sterilization can be carried out during the time after the molding of the part and prior to shipment to the medical institution by the company supplying the medical article or a third party contracted by the molder or at the medical institution prior to use in a medical procedure. Four common methods of sterilization are used for sterilizing plastic articles: ionization sterilization using ionizing radiation such as gamma or E-beam irradiation at dosages of 25–50 kGy, autoclaving using saturated steam at 121° C. to 130° C., dry heat sterilization and ethylene oxide gas sterilization. Other sterilization practice involves the use of multiple sterilization steps, where the above described common methods of sterilization are repeated or used in combination.
Polycarbonate is a particularly suitable thermoplastic employed in devices used by the medical industry because of its high reliability and safety benefits resulting from its optical transparency, toughness and heat resistance. Common medical devices made from polycarbonate include syringes, blood filter housings, intravenous connectors and parts for use in dialysis equipment among many others.
Polycarbonate articles are well known by those skilled in the art to perform well and resist discoloration after ethylene oxide gas sterilization and autoclaving sterilization. But when high energy irradiation sterilization methods are used, polycarbonate has a tendency to become discolored even though its mechanical properties have not generally been compromised (FIG. 1). Discoloration is undesirable for medical applications because it impacts the transparency of the article. The loss of transparency due in part to severe yellowing can severely damage the function and safety of the medical devices because monitoring of blood or other liquid flow may be severely hindered. Severe discoloration is also undesirable because medical devices with a compromised appearance can undermine the confidence of a patient undergoing treatment using such a polycarbonate medical device.
The discoloration of polycarbonate when treated with high-energy radiation such as gamma or E-beam radiation is thought to result from production of highly energetic and unstable species along the polycarbonate backbone. These species result in a yellow appearance and some loss of optical clarity of the polycarbonate article. The degree of yellowness and loss of optical clarity depends on the irradiation dosage.
The initial yellowness produced upon irradiation normally decreases with time and with further normal light exposure (FIG. 2). This color reversal is known by those skilled in the art as photobleaching. Eventually, the photobleached part reaches a steady state color shift, which is more yellow and less transparent than the part prior to initial irradiation, but less yellow and more transparent than immediately following initial irradiation. The length of time required to reach the steady state yellow color depends on the radiation dosage and the amount of normal room light exposure the part receives after the irradiation process.
A variety of additives have been developed to reduce the color shift in polycarbonate resulting from E-beam or gamma irradiation, including, but not limited to, bromine-containing additives, sulfide-containing additives, and polymeric and non-polymeric alcohols such as glycols. Also, blends of polycarbonate with more ionizing radiation stable polymers such as certain polyesters have been tried for this purpose. These solutions are effective to varying degrees to reduce yellowness and loss of optical transparency, but they can also adversely impact the physical properties and transparency of the polycarbonate article.
Prior to the invention of the compositions and methods described herein, polycarbonate compositions for the medical industry were well known for their tendency to undergo severe yellowing and loss of transparency when subjected to the high energy levels associated with ionizing radiation sterilization. As noted, these changes are highly undesirable in the medical industry. Ionizing radiation sterilization methods are becoming the preferred sterilization method for medical suppliers to hospitals. Therefore, it has been apparent that the continued use of polycarbonate resin in medical device applications (e.g., bottles, instruments, etc.) will be severely limited unless a composition and/or method can be found to mitigate the tendency of polycarbonate to yellow and lose transparency upon ionizing radiation sterilization.